The present invention relates to a method of thawing by high frequency heating, and, in particular, to a method of performing what is called semi-thawing in which the thawing is finished when the ice of the frozen food stuff is substantially melted.
In general, there are two methods of thawing, namely; thawing by external heating using heat conduction and thawing by internal heating using high frequency heating. Thawing by external heating is such that the surroundings of a food stuff is maintained at a higher temperature than the temperature of the food stuff so that heat flows into the frozen food stuff thereby increasing its enthalpy.
In this method, thawing of the frozen food stuff proceeds from the outer periphery of the food stuff toward the center thereof. This thawing by external heating is seen in thawing which utilizes an electric resistance heater, hot water or infrared ray lamp. Thawing by external heating generally requires a long time since most food stuffs are of low heat conductivity. When the outer layer of the food stuff is melted into water, its heat conductivity is decreased below that of the ice, and hence the rate of heat conduction into the internal part of the food stuff is decreased. If the food stuff is thawed in the same temperature zone as was used in freezing from initial to final temperatures, the thawing takes place in substantially twice the time required for freezing. The fact that the thawing takes a long time indicates that a difference in quality has occurred in the solid food even immediately after the thawing process.
In the case of thawing by high frequency electromagnetic waves, on the other hand, the food stuff is caused to generate heat to increase the enthalpy thereof. The thawing progresses from the outer surface and also from the interior of the food stuff since the frozen food stuff itself acts as a heat generating element due to the electromagnetic energy absorbed therein, and hence the food stuff is thawed very fast, resulting in the solid food stuff having a generally stable and uniform quality.
Although thawing by internal heating is superior to thawing by external heating, heating by conventional high frequency heating apparatus often result in unsatisfactory thawing in view of the fact that careful consideration is not given to the distribution of the high frequency waves, the size, shape and weight of the frozen food stuff to be thawed or the difference in the dielectric constants of water and ice.
In conventional high frequency heating apparatus, the frozen food stuff is thawed by (1) simply applying continuous radiation or by (2) repeating predetermined intermittent radiation without considering the weight of the food stuff. Since the dielectric constant of ice is low, the high frequency energy is not easily absorbed into the ice, and therefore in the case of the continuous radiation mentioned in item (1) above, the high frequency radiation first thaws the surface of the food stuff while the interior thereof is left frozen. This lack of uniformity in thawing condition leads to an aqueous solution existing only in the surface of the food stuff after a further certain time elapses.
The aqueous solution has a very high dielectric constant as compared with the frozen object and absorbs the high frequency energy more quickly, with the result that the surface of the food stuff is boiled while the interior thereof remains frozen.
In the case of the intermittent radiation mentioned in item (2) above, overheating of the surface of the food stuff is prevented during interruption of the high frequency radiation, and the heat in the surface is transmitted by conduction into the food stuff, thus allowing time for attaining uniform thawing of the whole food stuff. If the interruption time is too short, however, the disadvantage of the continuous radiation of item (1) is barely reduced and not resolved. If the interruption time of the high frequency wave radiation is too long, on the other hand, an unreasonably long thawing time is required undesirably, resulting in loss of advantages of the high frequency thawing.
Furthermore, since the time required for thawing with high frequency waves is short and the optimum thawing time varies with the quantity of the food stuff involved, it is very difficult to set the heating time to an optimum length.
In view of the above-mentioned disadvantages of the conventional high frequency heating apparatus, the object of the present invention is to solve the above-mentioned problems of the conventional high frequency heating apparatus.